Optical thin-films and optical elements comprising same

ABSTRACT

Optical thin-films are disclosed that are formed from optical thin films formed on a base plate arranged in a vacuum chamber. The base plates are held on a plurality of retaining frames of a base-plate retainer. The thin films are formed by heating the base plate and emitting a deposition material from a deposition source. The retaining frames are configured to make the entire base plate uniformly heated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/133,287, filed on Jun. 4, 2008, the contents of which areincorporated herein by reference in its entirety. This application alsoclaims priority to and the benefit of Japan Patent Application No.2007-148760, filed on Jun. 5, 2007, in the Japan Patent Office, which isincorporated herein by reference in its entirety.

FIELD

This disclosure relates to methods for forming optical thin-films and toapparatus for forming one or more optical thin-film layers on an opticalelement.

DESCRIPTION OF THE RELATED ART

In the manufacture of an optical element such as a lens, optical filter,or prism having a surficial thin film, certain optical thin-film-formingapparatus are conventionally used for forming the film. The conventionaloptical thin-film-forming apparatus includes a deposition dome having aspherical or planar shape. The dome holds a plurality of opticalelements mounted to the surface thereof inside a vacuum chamber. Theinterior of the vacuum chamber is heated as a layer of film-formingmaterial is deposited on the surfaces of the optical elements. Thedistance from the source of film-forming material to each opticalelement on the surface of the deposition dome varies. Hence, thedistribution of film-forming material as deposited is not uniform oneach optical element. I.e., the physical thickness ‘d’ of thefilm-forming material deposited on the optical elements varies dependingupon the position of the optical elements on the dome. This variabilityof the thickness ‘d’ causes changes the reflectivity or transmissivityof the optical elements.

To reduce the variability of the physical film thickness ‘d’, JapanUnexamined Patent Application No. 1998-280130 discusses rotating thedeposition dome during processing of the optical elements in theapparatus. Whereas rotating the dome reduces the variability of thephysical thickness of film material deposited on the optical elements,there is still notable variability of the optical thickness ‘nd’ of theindividual optical elements. The optical thickness ‘nd’ determinesimportant optical characteristics of each optical element, and is theproduct of (refractive index ‘n’)×(physical thickness ‘d’). That is,although the optical thin-film-forming apparatus disclosed in JapanUnexamined Patent Application No. 1998-280130 can form a film havinguniform physical thickness ‘d’ on the surface of the optical element, itcannot form a film having uniform refractive index ‘n’ over the surface;hence, the optical thickness ‘nd’ of the film is not uniform over thesurface.

For example, FIG. 6 is a graph showing the variability of the difference(“gap”) of the wavelength half-width (“half-value”) from the maximumwavelength value of an optical filter, formed with an optical thin filmon a crystal base plate in the conventional manner, as a function ofposition. In FIG. 6, data for two sizes of crystal base plates(including surficial thin films) are plotted, one having dimensions 58mm×60 mm size and the other having dimensions 40.5 mm×48 mm. In FIG. 6the vertical axis is the gap of wavelength half-value from the maximumvalue (the target half-value of the wavelength is zero, which wouldyield a zero gap), and the horizontal axis is distance (mm) from acentral area of the optical filter to regions of the filter surroundingthe central area.

Around the central area of the optical filter, the achieved wavelengthhalf-value is substantially equal to the target wavelength. But, in thesurrounding regions, the wavelength half-value is not equal to thetarget value because the surrounding regions have slightly differentproperties than the central area. I.e., in the surrounding regions,wavelength is lower than the designated wavelength. The resultingdifference is produced because, even though the physical thickness ‘d’is uniform across the base plate, the refractive index in thesurrounding regions is lower than in the central area, which reduces thewavelength in the surrounding regions. Although a conventional opticalthin-film-forming apparatus can make optical elements having a uniformphysical thickness ‘d’, the apparatus cannot make optical elementshaving a uniform physical thickness ‘nd’ over its entire surface. Inother words, the conventional optical thin-film-forming apparatus cannotsatisfactorily reduce the variability of the refractive index ‘n’ overthe surfaces of the optical elements.

In recent years, improvements in the quality of optical elementsincluding surficial optical thin films are strongly demanded ascorresponding improvements are made in the technical sophistication ofoptical devices. To address this demand, for example, whenever anoptical filter having dimensions 40.5 mm×48 mm is required, an opticalfilter comprising a thin film having uniform physical thickness ‘d’ on acrystal plate having dimensions 58 mm×60 mm is formed using aconventional optical thin-film-forming apparatus. After the filter inthe apparatus, peripheral regions of the 58 mm×60 mm plate (havingexcessively deviant refractive index ‘n’) are cut away to produce anoptical filter having dimensions of 40.4 mm×48 mm and exhibitingreasonably uniform optical thickness ‘nd’ overall.

In such a manufacturing method, larger optical elements (intended to beperipherally trimmed later) than required are attached to the depositiondome and processed. Thus, the numbers of optical elements that can beattached to the deposition dome are relatively few. Also, having to trimthe periphery of the optical elements after forming the thin filmswastes the peripheral areas of the optical elements and requires extrawork and higher costs.

The instant disclosure provides, inter alia, optical thin-film-formingmethods and apparatus for forming optical thin films having uniformoptical thickness ‘nd’ at both the central area and surrounding regionsof high-quality optical elements without having to trim peripheralregions of the optical elements.

SUMMARY

An optical thin-film-forming method according to a first aspect forms anoptical thin film on an optical base plate while the base plate issituated in a vacuum chamber. The base plate can have any of variousconfigurations not limited to planar and not limited to rectangular orcircular. The method comprises mounting the base plates on a pluralityof respective retaining frames of a base-plate retainer in a vacuumchamber. The base plates are heated while film-deposition material isemitted from a deposition source in the chamber. The retaining framesare configured to achieve substantially uniform heating of the entirebase plates during film-formation processing.

As discussed, conventional methods produce films having uniform physicalthickness ‘d’ but not having uniform optical thickness ‘nd’. Withmethods as disclosed herein, the temperature of the base plates iscontrolled during processing to produce thin films having substantiallyuniform refractive index ‘n’ over the surface of the base plate. Thus,thin-films are produced of which the optical thickness ‘nd’ issubstantially uniform from the central area to the surrounding regionsof the optical elements.

According to a second aspect, the retaining step in the method comprisesmounting the base plates on retaining frames made of the same materialas the base plates. By making the retaining frame of the same materialas the base plate, heat in the base plate can be substantially preventedfrom escaping from the surrounding regions of the optical elements tothe retaining frames. This configuration enables the entire base plateto be uniformly heated, which allows formation of thin films of whichthe optical thickness ‘nd’ is substantially uniform from the centralarea to the surrounding regions.

According to a third aspect, the step of retaining the base platescomprises mounting the base plates on retaining frames made of amaterial having lower thermal conductivity than the base-plateretainers. Thus, the retaining frames are made of a material havingrelatively low thermal conductivity. It was considered that the heat insurrounding regions of the base plate, and then the refractive index ‘n’of the film formed in the surrounding regions, are relatively lowbecause heat escapes from the base plates via the retaining frames tothe base-plate retainers. By forming the retaining frames of a materialhaving lower thermal conductivity than the retainers, the entire baseplate can be uniformly heated to ensure that the optical thickness ‘nd’is uniform from the central area to the surrounding regions.

According to a fourth aspect, the retaining step of the method comprisesmounting the base plates on retaining frames that include individualheaters. To such end, the retaining frames are individually heated toprovide heat as required to the surrounding regions of the base platesmounted to the retaining frames. Thus, the entire base plate can beheated uniformly, which achieves a substantially uniform opticalthickness ‘nd’ of the film from the central area to the surroundingregions. This aspect also allows manufacture of base plates havingelevated refractive index ‘n’ in the surrounding regions (compared tothe central area) by heating the surrounding regions to a temperaturehigher than the temperature of the central area during processing.

According to a fifth aspect, the retaining step comprises attaching, tothe retaining frame holding a base plate, a cover that covers thesurrounding region of the base plate. The cover is attached to theretaining frame on the side thereof opposite the side of the depositionsource. The cover retains the heat in the surrounding region of the baseplate, allowing the entire base plate to be uniformly heated. Thisaspect also allows manufacture of an optical element having elevatedrefractive index ‘n’ in the surrounding regions (compared to centralareas) by heating the surrounding regions to a higher temperature thanthe temperature of the central areas.

The optical element formed by any of the methods summarized above havesubstantially the same optical thickness ‘nd’ both at the central areaand in surrounding regions. Also possible is the formation, on opticalbase plates, of optical thin films having higher refractive index in thesurrounding regions than in the central area. If the refractive index ishigher in the surrounding regions than in the central area, the filtercan function at the same wavelength with rays passing through a lens inboth dimensions orthogonal to the optical axis.

Therefore, according to the invention, an optical thin film havingcontrolled (e.g., uniform) optical thickness ‘nd’ can be formed at thecenter and in surrounding regions of high-quality optical elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational section of an opticalthin-film-forming apparatus 10.

FIG. 2 is a plan view, from the under-side, of the deposition dome 19located inside the optical thin-film-forming apparatus 10.

FIG. 3A is a plan view of a first embodiment of a retaining frame 33Afor attaching two optical elements OE on one retaining frame.

FIG. 3B is a plan view of a second embodiment of a retaining frame 33Bfor attaching four optical elements OE on one retaining frame.

FIG. 4 is a flow chart of an embodiment of a method for making anoptical filter.

FIG. 5 is a graph of the spectral transmission of an infrared cut-offfilter configured as 1 mm of tested crystal plate having forty layers ofan optical thin film.

FIG. 6 is a graph which shows the difference (“gap”) in wavelengthhalf-value minus maximum wavelength passed by an optical filter,compared to a target value.

FIG. 7 is a graph showing the effect of heating temperature of a crystalbase plate on refractive index of an area including a TiO₂ film.

FIG. 8A is a plan view of an embodiment of the retaining frame 33Acomprising an electric heater 37.

FIG. 8B is an elevational section along the line B-B of FIG. 8A.

FIG. 9A is a plan view of an embodiment of the retaining frame 33including a heat-retaining cover 39.

FIG. 9B is an elevational section along the line B-B of FIG. 9A.

FIG. 10A is a graph showing the wavelength half-value (as a function ofposition) of a crystal base plate having size of 40.5 mm×48 mm,processed by an apparatus comprising individual heaters for theretaining frames.

FIG. 10B is a graph showing the wavelength half-value (as a function ofposition) of a crystal base plate processed by an apparatus comprisingindividual covers for the retaining frames.

FIG. 11 is a diagram showing that a beam arrives at a photoelectricconverter 45, comprising a CCD or CMOS sensor, from a convex lens viathe optical filter 43.

DETAILED DESCRIPTION

Exemplary Configuration of Optical Thin-Film-Forming Apparatus

FIG. 1 is an elevational section of an embodiment of an opticalthin-film-forming apparatus 10. The optical thin-film-forming apparatus10 of this embodiment comprises a first deposition source 17 and asecond deposition source 18 arranged off-center in the lower portion ofa deposition (vacuum) chamber 12. The chamber 12 also contains adeposition dome 19 that includes base-plate-retaining portions that holdoptical elements OE made of glass or crystal, for example. Thedeposition chamber 12 is evacuated to a suitable vacuum level, such asapproximately 10⁻⁴ Pa. The first deposition source 17 contains afilm-forming material such as TiO₂, which evaporates when it isirradiated by an energetic-ion beam. The second deposition source 18contains a material such as SiO₂, which evaporates when it is irradiatedby an electron beam.

Also located inside the chamber 12 are a first shutter 21, movable froman open position to a shielding position between the first depositionsource 17 and the deposition dome 19, and a second shutter 22, movablefrom an open position to a shielding position between the seconddeposition source 18 and the deposition dome 19. The apparatus 10 alsoincludes a control system (not shown) configured to actuate, in analternating manner, the first shutter 21 and the second shutter 22,depending upon the desired thickness and composition of the optical thinfilm to be formed on the optical elements OE. More specifically, thecontrol system opens and closes, in an alternating manner, the firstshutter 21 and the second shutter 22 as required to expose the sources17, 18 selectively to the energetic ion beams to form the films. Themovements of the first shutter 21 and of the second shutter 22 controlthe beginning and end of formation of each thin-film layer. The physicalfilm thickness ‘d’ is controlled with rotation locomotion of thedeposition dome 19 relative to the first deposition source 17 and thesecond deposition source 18.

A first diffusion panel 27 and a second diffusion panel 28 are arrangedabove the first shutter 21 and the second shutter 22, respectively, atpositions adjacent the surrounding area of the deposition dome 19 so asto form the physical film thickness ‘d’ on the optical elements OE in auniform manner.

The deposition dome 19 is supported and rotatable at about the centerline of the deposition chamber 12 via a driving motor 26 situated abovethe chamber 12. Also, a heater 24 is provided above the deposition dome19 inside the chamber 12. Radiant heat produced by the heater 24 heatsthe deposition dome 19 and the optical elements OE to a designatedtemperature, for example from 200° C. to 270° C. Whenever the energeticion beam is irradiated on the first deposition source 17 and/or thesecond deposition source 18, the temperature inside the depositionchamber 12 can be more than 280° C. Note that the deposition dome 19 maybe not only spherical in shape, but also planar or a stepped-domeconfiguration.

FIG. 2 is a plan view, upward from the under-side of the deposition dome19 inside the optical thin-film-forming apparatus 10. The depositiondome 19, as shown in FIG. 2, includes a plurality of rectangularopenings or voids 31 arranged in a concentric fashion. Each rectangularopening 31 has dimensions of, for example, 100 mm on a side. A retainingframe 33, configured to retain a respective optical elements OE, isattached to each rectangular opening 31. The deposition dome 19 is madeof a suitable metal such as stainless steel or steel.

FIG. 3 provides plan views of exemplary retaining frames 33. FIG. 3Ashows a first embodiment of a retaining frame 33A configured to hold twooptical elements OE. The retaining frame 33A defines a pair of firstvoids 35 through which evaporated material from the first depositionsource 17 or from the second deposition source 18 passes to the opticalelements OE being held by the retaining frame 33A. The optical elementsOE are made of, for example, glass or crystal. FIG. 3B shows a secondembodiment of a retaining frame 33B to which four optical elements OEcan be attached. The retaining frame 33B defines four voids 36 throughwhich evaporated material from the first deposition source 17 or fromthe second deposition source 18 passes to the optical elements OE beingheld by the retaining frame 33B. Although not shown, a retaining frame33 can be configured to hold only one optical element OE per retainingframe.

The retaining frames 33 can be configured to hold not onlyrectangular-shaped optical elements OE, but also round-shaped orother-shaped optical elements. In the depicted embodiment, thedeposition dome 19 and the retaining frames 33 are separate entities; inan alternative configuration, the deposition dome 19 itself isconfigured with rectangular voids 31 each with its own retaining frameto retain the optical elements OE.

Movements of the Optical Thin-Film-Forming Apparatus

Next, a method for making an optical filter comprising an optical thinfilm having forty layers is described with reference to FIG. 4. Themethod utilizes the optical thin-film-forming apparatus 10 describedabove. FIG. 4 is a flow-chart of the method, as applied to making anoptical filter. In step S11 the optical elements OE are attached torespective retaining frames 33, and the retaining frames 33 (withattached optical elements) are mounted to the deposition dome 19. Instep S12 regions inside the deposition chamber 12, especially thedeposition dome 19 and the optical elements OE, are heated to adesignated temperature by the heater 24 as the deposition chamber 12 isbeing evacuated to a desired vacuum level. In step S13 the depositiondome 19 is rotated at a desired angular velocity by the driving motor26. In step S14 the first shutter 21 and the second shutter 22 are intheir closed positions. In step S15 the first deposition source 17 andthe second deposition source 18 are heated by an energetic ion beam. Instep S16 the first shutter 21 is opened for a predetermined length oftime to deposit material from the first deposition source 17 as a firstthin-film layer on the optical elements OE. After the designated time,the first shutter 21 is returned to the closed position. Thus, the firstlayer of the film is formed, and the method progresses to the next stepS17. In step S17 a determination is made of whether the designatednumber of layers has been deposited on the optical elements. If only thefirst layer has been formed, the process advances to step S18 via stepS15. In step S18, material from the second deposition source 18 isdeposited as a second thin-film layer on the optical elements OE. Tosuch end the second shutter 22 is opened and left opened for apredetermined length of time, then closed. Thus, the second layer of thefilm is deposited. Then, the method advances to the step S17. Byrepeating steps S16 and S18 as required, the desired thing-film-coatedoptical elements are produced, such as optical filters on which the thinfilms comprise forty layers.

Uniformizing ‘nd’ Across the Optical Thin Film

The optical thin films provide the optical elements OE with acharacteristic denoted ‘nd’, or “optical thickness,” which is a productof (refractive index ‘n’)×(physical film thickness ‘d’). If the physicalfilm thickness ‘d’ is uniform across the optical element, if therefractive index ‘n’ decreases across the optical element, then theoptical-film thickness ‘nd’ correspondingly decreases across the opticalelement. For example, if the optical-film thickness ‘nd’ of an infraredcut-off filter is decreased in a particular region of the filter, thenthe wavelength range of IR radiation passed by the filter is alsodecreased.

FIG. 5 is a graph of the spectral transparency of an exemplary infraredcut-off filter, of which 1 mm of the crystal plate having forty layersof an optical thin film was tested. The vertical axis is transparencyand the horizontal axis is wavelength (350 nm to 1200 nm). Thesolid-line plot is of transparency of the central area of the infraredcut-off filter, and the dashed-line plot is of transparency ofperipheral regions of the infrared cut-off filter (surrounding thecentral area of the filter). As can be seen, the transparency of theperipheral regions is shifted to shorter wavelengths, compared to thecentral area.

FIG. 6 is a graph showing of the difference (“gap”) of wavelengthhalf-values from maximum wavelength exhibited by two different sizes ofoptical filters, relative to respective target values. The filters eachcomprise a thin film on a crystal plate. In FIG. 6 the vertical axis isthe difference (“gap”) of wavelength half-value (target half-value iszero), for wavelengths providing 50% transparency in FIG. 5.

The horizontal axis is position (mm) from the central area to theregions of the optical filter surrounding the central area. With bothcrystal base plates having dimensions of 58 mm×60 mm and 40.5 mm×48 mm,respectively, the wavelength half-values exhibited at the surroundingregions progressively decrease and are lower than the line of −2 nm inthe far-surrounding regions.

FIG. 7 is a graph showing the effect on dispersion by a TiO₂ layer on acrystal base plate to various wavelengths at different temperatures.Index of refraction (‘n’) is the vertical axis and wavelength is thehorizontal axis. This graph shows that the refractive index of TiO₂increases with increases in process temperature. More specifically, atany specific wavelength, increasing the process temperature produces acorresponding increase in refractive index ‘n’ of TiO₂. From this datathe inventor discovered that the optical thickness ‘nd’ can be madesubstantially uniform by appropriate control of process temperature inthe surrounding regions versus central area, e.g., by maintaining thetemperature of the surrounding regions of the optical element OE thesame as the temperature of the central area of the optical elementduring thin-film formation. If the temperature of the heater 24 issimply raised in the conventional optical thin-film-forming apparatus10, the temperature of the central area of the optical element OE iscorrespondingly increased, but the temperature remains non-uniformacross the optical element. Therefore, the following embodiment achievesuniformity of the temperature of the surrounding regions and centralarea of the optical element OE (or even achieves slightly highertemperature of the surrounding regions compared to the central area, ifdesired).

In the first embodiment, the retaining frame 33 shown in FIG. 3 is madeof the same material as the optical element OE. Conventionally, theretaining frame 33 is made of the metal used to form the deposition dome19, such as stainless steel or steel. However, in this embodiment, theretaining frame is made of the same material as the optical elements OEso as to have the same thermal conductivity as the optical elements.More generally, the retaining frame can be made of a material havinglower thermal conductivity than stainless steel or steel to ensure thatsome heat in the surrounding regions of the optical element OE escapesthrough the retaining frame 33 to the deposition dome 19. For example,the thermal conductivity of stainless steel having low thermalconductivity is 15 W/mK whereas the thermal conductivity of glass is 1W/mK and of crystal is 8 W/mK.

Using a retaining frame 33 made of the same material as the opticalelement OE, an infrared cut-off filter was made according to the methoddiagrammed in FIG. 4. FIG. 10A is a graph of the wavelength half-valueexhibited by a crystal base plate having dimensions of 40.5 mm×48 mmformed in this manner. Note that the curve in FIG. 10A differssignificantly from the curve shown in FIG. 6 produced by aconventionally formed crystal base plate of the same size (40.5 mm×48mm). In FIG. 10A the wavelength half-value is substantially constantboth at the central area and at the surrounding regions, the half-valuebeing only about 1 nm lower at the outer edges of the surroundingregions.

In a second embodiment, a retaining frame 33 as shown in FIG. 3 is madeof zirconia ceramic material. This embodiment of a retaining frame 33has a microporous structure and exhibits heat resistance. Thus, verylittle heat in the surrounding regions escapes to the deposition dome 19via the retaining frame 33. The thermal conductivity of zirconia ceramicmaterial having microporous structure is 0.2 W/mK. Thus, a retainingframe 33 formed of this material has lower thermal conductivity than thedeposition dome 19 made of a metal such as stainless steel. The thermalconductivity is preferably less than 1.0 W/mK.

Using a retaining frame 33 made of zirconia ceramic material, aninfrared cut-off filter was made using the method diagrammed in FIG. 4.This infrared cut-off filter exhibited the nearly flat curve ofwavelength half-values as shown in FIG. 10A. That is, the wavelength ofthe infrared cut-off filter does not significantly decrease even at thesurrounding regions.

In alternative embodiments, the retaining frame 33 is made of soda glassor crown glass instead of the zirconia ceramic material.

In a third embodiment, as shown in FIG. 8, an electric heater 37 isarranged around voids 35 in the retaining frame 33A. FIG. 8A is a planview of the retaining frame 33A including the electric heater 37, andFIG. 8B is an elevational section taken along the line B-B of FIG. 8A.The retaining frame 33 of this embodiment can be made of a metal such asstainless steel or steel since the electric heater 37 heats theretaining frame 33 as desired. Meanwhile, the deposition dome 19 isheated by the heater 24 to a temperature of 200 to 270° C. As a result,the inner temperature of the deposition chamber 12 may be higher than apreset temperature as the beam of energetic ions heats the firstdeposition source 17 and the second deposition source 18. The electricheater 37 heats the retaining frame 33 to a temperature of 270 to 310°C. The heater 37 includes a power cord 37A that can be connected to anadjacent heater (e.g., in an adjacent retaining frame) and finallyconnected to a power source.

An infrared cut-off filter was made by a method according to theflow-chart shown in FIG. 4, using the retaining frame 33A with theelectric heater 37 to adjust the temperature of the optical element.Examples of this infrared cut-off filter exhibited substantially thesame distribution of wavelength half-value as shown in FIG. 10A.

If the infrared cut-off filter is formed by adjusting the temperature ofthe electric heater 37 so that the temperature of the surroundingregions of the optical element OE is higher than the temperature of thecentral area, a wavelength half-value distribution as shown in FIG. 10Bis produced. That is, the filter producing the data shown in FIG. 10Bhas a slightly higher wavelength half-value in the surrounding regionsthan in the central area. Since the physical film thickness ‘d’ issubstantially uniform across the optical element, the refractive index‘n’ of the deposited film material progressively increases from thecentral area to the surrounding regions.

In a fourth embodiment, as shown in FIG. 9, a heat-retaining cover 39 issituated around the first void 35 of the retaining frame 33A. FIG. 9A isa plan view of the retaining frame 33 including the heat-retaining cover39, and FIG. 9B is an elevational section along the line B-B of FIG. 9A.The retaining frame 33A of this embodiment is made of a metal such asstainless steel or steel. The heat-retaining cover 39 is made of thesame material. The heat-retaining cover 39 is situated on the oppositeside from the first deposition source 17 or the second deposition source18, i.e., on the side of the heater 24. Each heat-retaining cover 39defines a void 39A; the heat of the optical element OE in the area ofthe void 39A is not retained. That is, heat in the central area of theoptical element OE is not retained by the cover. It is difficult todispose of the heat of the optical element OE adjacent theheat-retaining cover 39, which allows the temperature around thesurrounding region of the optical element to become at least equal tothe temperature in the central area.

Using a retaining frame 33A with the heat-retaining cover 39, aninfrared cut-off filter was made based by a method as diagrammed in FIG.4. This infrared cut-off filter exhibited almost the same profile ofwavelength half-value shown in FIG. 10 by controlling the size of thevoid 39A and the thickness of the heat-retaining cover 39. If theinfrared cut-off filter is formed using a heat-retaining cover 39 havingsmall voids 39A, as shown in FIG. 10B, the infrared cut-off filterexhibits higher wavelength half-value at the surrounding regions than atthe central area of the filter.

Optical Filter Having Thick Optical Film Thickness ‘nd’ in SurroundingRegion

The third and fourth embodiments allow infrared cut-off filters havinghigher wavelength half-values at the surrounding regions than at thecentral areas to be manufactured. When such an infrared cut-off filterreceives a beam irradiated via a lens, it can be used as an effectiveinfrared cut-off filter.

FIG. 11 is a conceptual figure showing that a beam arrives at aphotoelectric converter 45 (CCD or CMOS) from a convex lens via anoptical filter 43. The rays passing through the convex lens 41 arerefracted relatively more in the surrounding regions and relatively lessin the central area. Whenever the incoming beam is not verticallyincident, it is shifted to a short-wavelength side in proportion to theincidence angle. Consequently, if the optical filter 43 has a largerwavelength half-value in the surrounding regions than in the centralarea, as shown in FIG. 11, then the beam propagating to thephotoelectric converter 45 can enter the photoelectric converter withthe same wavelength both at the surrounding regions and the centralarea. This allows the photoelectric converter 45 to achieve the samecolor sensing in both surrounding and central areas.

Note that the technical scope of present invention is not limited to theforegoing embodiments, and various changes may be made to theembodiments without departing from the scope of the invention. Forexample, in the described embodiments, film-forming is performed byvacuum-deposition. But this is not intended to be limiting. For example,film-forming can be performed to produce a combination of layers oforganic material and inorganic material by the RF ion-plating method. InRF ion-plating, an inorganic material is evaporated by vacuum heating togenerate scientifically active ions or particles of an organic andinorganic material by plasma discharge. Both materials can be combined.An organic material is introduced in a carrier gas such as an inert gas(e.g., argon or helium) and an inorganic/organic combined film iscreated by the energy of the plasma.

Also, the disclosed embodiments were described in the context of aninfrared cut-off filter. This is not intended to be limiting. The filtercan be another type of optical filter. The present invention also can beapplied to a lens or prism having a glass or crystal base platecomprising an optical thin film.

What is claimed is:
 1. An optical element, comprising: an optical thinfilm formed by mounting base plates in a vacuum chamber on a pluralityof retaining frames on a retainer holder; controlling a heatdistribution in the base plates from central areas to surroundingregions thereof; and while the base plates are mounted to the retainerholder and while controlling the heat distribution so as to besubstantially uniform from the central areas to the surrounding regionsof the base plates, emitting a deposition material from a depositionsource and depositing the deposition material on the base plates; andthe optical thin film having a distribution of optical thickness, over asurface of the optical element from a central area to a surroundingregion, in which the optical thickness in the surrounding region is atleast equal to the optical thickness in the central area.
 2. The opticalelement of claim 1, wherein the optical thickness in the central areaand surrounding regions is substantially equal.
 3. The optical elementof claim 1, wherein the optical thickness in the surrounding regions isgreater than the optical thickness in the central area.
 4. An opticalelement, comprising: a base plate having a surface including a centralarea and a surrounding region; and an optical thin film on the surface,the optical thin film in the surrounding region having a refractiveindex that is at least equal to the refractive index of the optical thinfilm in the central area.
 5. The optical element of claim 4, wherein theoptical thin film in the surrounding region has a refractive index thatis greater than the refractive index of the optical thin film in thecentral area.